EHA YEARBOOK 2009
general, the susceptibility depends on the specific end
point, the level of exposure, and the lengths of exposure.
In particular, those with chronic cardiopulmonary
disease/asthma/influenza are affected by short term/
moderate exposures, while long term/repeated exposure
results in an increased risk of mortality in a broad based
cohort of adults and children.
Health effects due to inhalation of particles:
ultrafine particles
The potential hazards from the inhalation of ultrafine
particles by humans are very different from those
from the inhalation of larger particles, since they are
inhaled into much deeper regions of the lung and they
are not readily removed from the airstream of inhaled
air in the upper parts of the respiratory tract. When
deposited in the small containments of the alveoli
region, diffusional deposition of the particles on the
epithelium becomes an efficient physical mechanism.
Alveolar deposition of 0.05 um particles is about 40%,
compared to about 10 % for 0.7 um particles (Maynard
2000).
If these particles are charged, they pose an added
risk to human health, since inhaled charged particles
have a five to six-fold increased probability of lung
deposition than uncharged particles of the same size
(Cohen & Xiong 1998). The nanoparticles deposited
in this oxygen/blood exchange region can penetrate
into the blood stream very quickly and efficiently. All
of the studies conducted thus far demonstrate that the
primary determinant of the effect of ultrafine particles
is their number and their surface area, not the weight
of the particles present (Morawska et al. 2003). This
means that the traditional use of particulate matter
weight measures is inappropriate when evaluating the
likely biological effects of ultrafine particulates.
It should, however, be kept in mind that it is not only
the size by which ultrafine and nanoparticles differ
from larger particles, but there are also substantial
differences in particle chemical properties, and thus in
the toxicological and carcinogenic effects they cause.
As shown by recent literature reviews (Morawska et
al. 2003; Morawska et al. 2007), there have been only
a relatively small number of epidemiological studies
conducted thus far. A total of only five international
epidemiological studies on ultrafine particles have been
conducted since 2003 (eight were conducted prior to
this time).
The current state of knowledge on the health effects
of ultrafine particles can be summarised by saying
that the array of epidemiological studies conducted
so far does suggest that exposure to ultrafine particles
is associated with respiratory and cardiovascular
effects, which holds true despite considerable gaps in
knowledge and some inconsistencies found between
different studies. While both fine and ultrafine
particles appear to affect health outcomes, such as
respiratory and cardiovascular morbidity and mortality,
they appear to do so independently of each other.
Fine particles show more immediate effects, while
ultrafine particles show more delayed effects on
mortality. However, at present, the database is too
limited (both in terms of number of studies and
number of subjects) and geographically restricted,
to allow clear conclusions on the mode of action or
generalisation to other settings.
Further studies are currently under way but more
studies in other settings need to be initiated to
improve our understanding of ultrafine particles and
health outcomes. The reviews showed that there are
several significant deficiencies of the epidemiological
studies conducted thus far, which can be summarised
as follows:
n First, all studies drew on data from central
monitoring stations to estimate the levels of
ultrafine particles that participants in the study
would have been exposed to. It is now understood
that in most cases, this did not accurately represent
real exposure levels and that monitoring of ultrafine
particles must be done at the location where the
exposure occurs. As explained above, exposure to
particle number concentrations in the proximity to
a road for example, can be up to 10 times higher
than away from the road.
n Second, the background concentrations of ultrafine
particles in the various cities investigated were not
sufficiently different to allow conclusions to be
drawn across the studies.
Some of the reasons for these deficiencies include:
n a lack of scientific understanding of the nature and
dynamics of ultrafine particles at the time of the
studies
n limited instrumental methods or capability to
monitor adequately exposure to ultrafine particles
n the absence of an interdisciplinary approach
Ultrafine Particles: Future Directions
Given that there is a poor correlation between ultrafine
particles (measured by number) and fine particle mass,
observed statistical independence (in the multiple
regression models) is of significance. Further, given
that fine and ultrafine particles often originate from
common sources, have different dynamics of particle
formation and accumulation and also display different
observed lead-lag relationships between exposure and
observed health responses, it is currently difficult to
make strong inferences about independent effects,
based on the epidemiological evidence provided so
Air Quality and its impact on Health: (continued)